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3/16 keyway in grade 8 bolt

I tried to cut a 3/16 inch keyway in a grade 8 bolt today 1900 rpm plenty of oil about 20 th deep no luck . I ordered some carbide end mills will they work? Am I running to fast? Did I shoot myself in the foot trying to use grade 8 bolts?
Richard

Try a different bolt. Bolts don't have a strict spec on alloy or hardness. They only need to meet a strength specification. I have no trouble machining grade 8 with HSS although I usually use carbide. But, every once in a while I run into a hard one that must have been from a different melt. Even grade 11 cap screws are perfectly machinable. Strength and hardness are not directly tied together.

Don't try applying that "rule of thumb" to cast iron for example. The stress/strain curve for metals is generally non-linear. Only in the elastic deformation part of the curve is it linear. Beyond that the hardness may increase due to strain hardening while the ultimate tensile strength goes down.

If the item in question has undergone any sort of cold work then the relation between hardness and tensile strength has been altered. That applies to cold drawn products as well as any product that has been cold formed such as a rolled and headed bolt.